FIELD OF THE INVENTION
The invention relates to a premixing chamber for mixing a compressed gas, such as compressed air, with a reducing agent, such as urea solution or ammonia water, for an exhaust gas purification system. The invention also relates to a nebulizing device for an exhaust gas purification system having a premixing chamber.
The premixing chamber is used, in particular, in an exhaust gas purification system fitted with a regulated or controlled diesel catalytic converter (CDC) in an internal combustion engine. It can also be used in an exhaust gas purification system for stationary diesel engines, e.g. up to 1000 kW mechanical power.
The principle of the regulated or controlled diesel catalytic converter (CDC) has proved to be an expedient method of reducing pollutants, especially the oxides of nitrogen, contained in the exhaust gas from an internal combustion engine. That method is applied primarily in internal combustion engines which are operated with excess air, such as in diesel and lean-burn engines, for example. That method, which is based essentially on the principle of selective catalytic reduction (SCR), has in the meantime been disclosed in numerous publications such as, for example, in German Published, Non-Prosecuted Patent Applications DE 43 09 891 A1, corresponding to U.S. application Ser. No. 08/490,115, filed Jun. 12, 1995; DE 43 10 926 A1; and DE 43 15 278 A1, corresponding to U.S. application Ser. No. 08/551,791, filed Nov. 7, 1995. In the SCR method, the oxides of nitrogen are contacted, together with ammonia, on a selective catalyst and are converted there to environmentally non-harmful nitrogen and water.
Due to the risk associated with the use of ammonia, namely toxicity, and due to the noxious odor ammonia causes, ammonia as such should not be carried in the vehicle if the internal combustion engine is fitted with a CDC system. Therefore, a reducing agent that is necessary for the catalytic conversion of the oxides of nitrogen is transported in the vehicle in the form of an aqueous urea solution. In each case the ammonia is produced by hydrolysis from that aqueous urea solution in the quantity required at any given moment to convert the oxides of nitrogen. In stationary smoke gas purification systems, e.g. downstream of power plants, pure ammonia or ammonia water can be used.
In accordance with German Published, Non-Prosecuted Patent Application DE 44 17 238 A1, provision is made for the exhaust-gas conduit of a diesel engine of a truck to be led directly up to the side of a cylindrical inlet chamber in which a funnel-shaped perforated metal sheet is disposed. An injection valve which is provided at the narrowest point of the funnel is used to inject an aqueous urea solution into an interior space of the funnel. In that way, a homogeneous distribution of the urea solution in the exhaust gas is achieved over the entire cross section of the inlet chamber. Adjoining the inlet chamber are a hydrolysis catalyst, a DeNO.sub.x catalyst and, if appropriate, an oxidation catalyst.
A solution which differs from the above is described in Published European Patent Application 0 586 913 A2 and is used as a basis in the introduction. A mixing device or premixing chamber is provided in order to obtain sufficient atomization of the medium to be nebulized, i.e. urea solution as the reactant, before it is introduced into the flow of exhaust gas containing pollutants. The medium and a compressed gas such as air, are introduced into the premixing chamber for intimate mixing with one another, i.e. to form an emulsion. The premixing chamber is connected through the use of a single mixing conduit or pipe to a nebulizing nozzle which, in turn, is disposed in the flow of exhaust gas. In the mixing chamber according to FIG. 2 of that publication, the compressed gas impinges laterally or tangentially on the flow of medium. It is possible to achieve quite a good distribution of the reducing agent liquid throughout the flow of exhaust gas with that premixing and spraying-in. In that case, the use of the compressed air also brings about cooling of the injection nozzle located in the hot flow of exhaust gas. However, a problem can occur when using the nebulizing device for spraying in an aqueous urea solution for the reduction of NO.sub.x, because that may cause crystals of urea and exhaust gas residues to be deposited in the process. In order to eliminate that disadvantage, it is proposed in that publication to cover the surface areas of the nebulizing device which come into contact with the urea with a catalytic coating. In the case of the nozzle, platinum coating and/or heating-up are also considered in addition. However, those aids are comparatively expensive.
The premixing chamber is particularly critical with regard to deposits in the nebulizing device. It has actually been shown that dry urea is deposited in the air conduit connected to the side of the premixing chamber for the purpose of tangential impingement, with the urea leading to the air conduit becoming blocked, specifically just before it opens into the premixing chamber, in particular, in the case of prolonged uninterrupted operation. The changed pressure conditions thus caused in the premixing chamber result in incorrect metering of urea solution. That must definitely be avoided.
A premixing chamber for producing a homogeneous mixture of a compressed gas and a reducing agent is also known from German Published, Non-Prosecuted Patent Application DE 42 21 363 A1. In that case, a feed pipe for the reducing agent projects perpendicularly to the flow direction of the compressed gas into a feed pipe for the compressed gas, with the feed pipe being widened in a funnel shape toward the outlet. However, that does not prevent the reducing agent from also flowing out counter to the flow direction of the compressed gas and forming deposits there. Such a premixing chamber, operating on the principle of a spray gun, is also not very efficient.